15+ Important Javascript Interview Question and Answers (2024)

One of the most widely used programming languages is JavaScript, which is used to create the majority of websites nowadays. JavaScript is mostly used for developing server-side apps, cross-platform desktop and mobile applications, interactive Web applications, and other applications. The need for qualified JavaScript developers has grown along with JavaScript’s increasing popularity.

Important Javascript Interview Question and Answers (2024)

All big tech giants require expert javascript developers to handle and create new interactive websites, and to crack those jobs, one should be proficient in developing javascript programs. The best way to prepare is to work on modern Javascript technologies, and what questions does most of the company ask?

In this article, we will provide you with some of the most commonly asked JavaScript interview questions that will help you get placed with big tech giants.

Check this Out:- 100+ JavaScript Projects For Beginners With Source Code

Q1 What is Javascript and what are its key features?

Ans: Javascript is a client-side, dynamic programming language. The main purpose of the high-level language Javascript is to create dynamic and interactive webpages.Using javascript we can build several unique and visually appealing websites that provides a great user interactivity. The key features of JavaScript are that it is a cross-platform language, follows OOPS concepts, and renders at the browser side to provide interactivity and reduce loading time.

Some common examples of interactive websites

Q2. List some features of javascripts?

Ans: The main features of javascript are:

1. Object oriented
2. Easily connect with HTML & CS
3. Support cross platform.
4. open source.
5. Highly Secure
6. Mutliple Libraries.

Q3 Which symbol is used for adding comments in Javascript?

Ans: Additional information about the code is provided inside comments, which are written within the code itself. But the compiler does not really execute these lines. Statements are kept from executing by using comments. When the compiler is running the code, comments are ignored. In JavaScript, comments are represented by one of two types of symbols:

1. Double Slash: (//) : This type of comment is used for commenting single line. We just need to add two forward slash in front of the text to add a single line comment.

// This is a single-line comment
let x = 5; // Variable assignment

2. Slash with Asterisk: This method is used for commenting multiple lines at a time. Using slash with Asterisk we can comment multiple lines as much as we want.We need to add a forward slash with asterisk symbol before and after the text to comment that particular section.

/*
  This is a multi-line comment
  It can cover several lines of code
*/
let y = 10;

Q4 Explain the difference between let, const, and var.

Ans:

Q5 What are the different datatypes present in the javascript?

Ans: Javascript has different data types that help developers organize and manipulate data in their programs.

Primitive Data types:

• String : String Represents the text that are closed inside the single or double quotes.

let welcome = "Hello, World!";

• Number: represents numeric values, including integers and floating-point numbers.

let age = 25;
let pi = 3.14;

• Boolean: Boolean represent the condition or logical data either true of false.

let isStudent = true;

• Undefined: This data type define uninitialized or missing values.

let undefinedVariable;

• Null: Null datatypes represent invalid values or blank values.

let nullValue = null;

Non-Primitive Data types: In JavaScript, reference types are another name for non-primitive data types. Non-primitive types are modified and saved as references as as compared to primitive types, which are immutable and stored directly in memory. Only one value can be stored in primitive data types. Non-primitive data types are used to hold many complex values.

Object: A collection of data is stored in an object.It is a set of key-value pairs, where the values can be any kind of data, including other objects are stored.

let person = {
  name: "John",
  age: 30,
  isStudent: false
};

Array: To store a group of values, one type of data structure is an array. When creating an array, square brackets are used to contain items of various data types “[]”.

let colors = ["red", "green", "blue"];

Function: Function is a block of code that perform some particular operation or group of operation on the website. Function is ablock of code that can be modified to change the function operations.

function addNumbers(a, b) {
  return a + b;
}

Q6 What is an operator in javascript?

Ans: In JavaScript, operators are keywords or symbols that carry out function on variables.Operators are the variable that helps user to carry out different function. Using operators user can perform operation such logical , artihmetic  and comparison operators.

Q7 What are the different types of operators in javascript?

Ans: The different types of operators in javascript are:

Arithmetic Operators: Arithmetic operators are the operators which perform out function that are mostly used by a developer while creating a program . Using arithmetic operators we can perform operations such as “+”,”-“,”*”,”/”

Addition: This operator is used to add two given variables and are usually denoted by “+” . 

let sum = 5 + 3; // sum is 8

Subtraction: The subtraction operator is used to subtract the smaller variable from the larger variable to produce a positive result. The subtraction operator is denoted by “-“.

let difference = 10 - 4; // difference is 6

Multiplaction(*) : This operator is used to multiple two operands or variables.

let product = 3 * 4; // product is 12

Assignment operators: Assignment operator, as the name suggests, is used to assign value to different variables. This operator is denoted by “=”, equal to signal, which helps the user assign the values to the variables.

let x = 5;

Comparison operators: A comparison operator is used to compare two variables and print the result in the boolean, either the value is true or else the value is false. This operator is used to compare whether the value of any variable is true or false.

Comparison operators are of two types:

Equal(‘==’): This operators checks if the two are equal or not.

let isEqual = 5 == '5'; // isEqual is true (loose equality)

StrictEqual(‘===’): This operator not only checks whether the values are equal; it also checks whether the given value is of the same type or not.

let isStrictEqual = 5 === '5'; // isStrictEqual is false (strict equality)

Conditional operators: The two conditions are checked using conditional operators, sometimes called ternary operators (if-else): if the condition is true, the action under the if condition is carried out; if not, the operation is carried out.

let isAdult = age >= 18 ? 'Adult' : 'Minor';

Q8 What is the difference between “==” and “===” operators?

Ans:

here below are some example of equal to and strictly equal to operators:

console.log(5 == “5”); // true (type coercion)
console.log(5 === “5”); // false (strict equality)
console.log(1 == true); // true (type coercion)
console.log(1 === true); // false (strict equality)
console.log(null == undefined); // true (type coercion)
console.log(null === undefined); // false (strict equality)

Q9 What is Hoisting in javascript?

Ans: Hoisting is the process of shifting any declaration—such as a variable or function declaration—to the top of the script or function before any code is run.

Currently, when a variable is declared using var, it is hoisted and assigned the default value undefined until it is actually assigned later in the code. On the other hand, if you use let or const, they likewise get hoisted, but until you actually give a value to them in the code, they remain uninitialized (no default undefined value).

console.log(x); // undefined
var x = 5;

// This is equivalent to:
var x;
console.log(x); // undefined
x = 5;

Q10 what is the use of the isNaN function?

Ans: JavaScript’s number isNan function is used to determine whether a value is of type “Number” and is NaN (not a number). JavaScript’s isNaN function can be used to determine if a given integer is NaN (Not a Number) or can be changed into a number that is acceptable. If the value is NaN or cannot be changed into a valid number, it returns true; if not, it returns false.

Q11 What is ‘this’ keyword in javascript?

Ans: In JavaScript, this keyword refers to the function’s execution in which the current code is being executed. This can relate to the global object, a particular object (when a function is a method of that object), a newly created object (when a function is used as a constructor), or it can be set manually using call(), apply(), or bind() methods, depending on the context.                                                                                                            

Q11 Define Closure?

Ans: A closure occurs when a function is combined (contained) with reference to its external state (the lexical environment).Closures are required when a variable specified outside the scope of the reference is accessed from an inside scope.

function outerFunction() {
  let outerVariable = "I am from the outer function";

  function innerFunction() {
    console.log(outerVariable);
  }

  return innerFunction;
}

const closureFunction = outerFunction();
closureFunction(); // Outputs: I am from the outer function

Q12 What is DOM? What is the use of document object?

Ans: DOM Stands for document object model. A document object represents the HTML document. ​ In simpler terms, the DOM provides a way for programs (like JavaScript) to interact with the structure, style, and content of HTML or XML documents.It can be used to access and changeHTML text.

if you want to know the in-depth concept about DOM then follow thw below link

Q13 What is the scope in javascript?

Ans: A scope is the part of code that gives us access to particular variables and functions. It chooses which variables and functions are displayed. Functions and variables declared inside a certain scope are not available outside of it.

Q14 What is callback function?

Ans: Callback functions are those that are sent as parameters to other functions. To get the intended outcome, we will use the callback function that is part of the function.

Q15 What are the three types of javascript error?

Ans:

Syntax Error: A syntax error is generated when the program is not built correctly.
Reference Error: The javascript function that is unable to find the reference to the variable in the memory ID is called a reference error.
Type Error: This type of error is generated when we try to reassign the value to the const variable.

Conclusion

These are the top 15 JavaScript interview questions that were covered in this article. These interview questions will help you accomplish the technical part to land a dream job as a JavaScript developer.

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Author : Arun